In known machine tool design, it has been necessary to devise joints incorporating a mechanism able to constrain the relative rotation between two struts whilst permitting them to pivot on a common axis, thus allowing the couplet to be free to articulate in all axes. These types of joints are necessary for building parallel strut structures, such as hexapods.
Methods have been proposed where the universal freedom about a pivot point is enabled in the form of a ball and socket joint; where each strut is connected to a hemisphere together defining the ball of said joint. However, these previous solutions to this problem have proved difficult to build accurately because of the requirement to assemble an accurate sphere out of two hemispheres. It is much easier to build a single surfaced sphere which can be ground and/or lapped to achieve the degree of sphericity required.
Modern machine design such as hexapod based machine tools may require universal joints capable of a very wide degree of freedom.
A ball and socket joint is conceptually a good solution because it can be made to high precision and is easy to calibrate because it accurately maintains its pivot origin. The disadvantage of a conventional solution is the limited degree of freedom that can be supported because of the necessity of holding the ball over a large angle to minimise holding pressures and hence friction.
One solution that has been proposed is to retain the ball magnetically in the bottom half of the socket only, thereby leaving more than a hemisphere of articulation space. While being suitable for some applications, the joint would become unduly massive to achieve the very high holding forces necessary in some cases such as for large scale machining.
In the design of hexapods, it has been necessary to provide a mechanism well suited to drive a screw shaft through a focal point notionally within a two axis universal joint.
To build a ballscrew drive point in a universal joint it is necessary to constrain its motion to two degrees of freedom so that it can resist the torque of rotating the ballnut. This can currently be achieved either by a standard two axis gimbal with orthogonally opposed axes or by a suitably constrained ball and socket joint. Both of these methods have disadvantages.
In the case of the standard gimbal, because preload needs to be applied to each end stop, any thermal or load stresses will move the pivot point about both orthogonal axes. This makes the node point unreliable.
In the case of the constrained ball and socket joint, when confronting large loads, stiffness can only be achieved at the cost of friction in the ball to socket interface; and/or extending the socket over more of the ball thereby limiting the articulation.
There is a need for a mechanism able to translate a load through six degrees of freedom at extreme resolution. On many occasions it is desirable to alter the position and alignment of instrument components by very small degrees. Presently this is accomplished by multi-axis stages arranged in a series chain of polar and Cartesian single axis stages. It is difficult to keep such an arrangement stiff without unduly increasing the size and weight of the structure and reducing its freedom of available movement.